My phone storage was full and I couldn't open WhatsApp. I put the phone on charging and the next day when I tried to open WhatsApp, I was logged out. I had to provide my phone no and other information to login as we do when we change our device. Also, all of my WhatsApp data was gone, messages, history and pictures. Although, the saved media was still available via the Photos app. Was my WhatsApp account hacked, or did iOS deleted the data?
Also offload unused apps setting (available under
Settings app -> General -> iPhone Storage)was not enabled. I am using an iPhone 5S 16GB.